In the operation of a.c. superconducting magnets formed from cabled superconducting wires, energy losses due to coupling can be a major problem. Coupling is essentially a short-circuit between adjacent strands in a cable, caused by insufficient resistance at the interfaces of the strands. Induced currents flow across the interfaces of coupled strands, dissipating energy.
For Nb.sub.3 Sn cables, which must be reacted after magnet winding due to the brittleness of the Nb.sub.3 Sn intermetallic, coupling can be a particularly acute problem. Nb.sub.3 Sn strands usually have a copper surface. Since the reaction temperature is in the 700.degree. C. range, the Nb.sub.3 Sn strands tend to sinter together, coupling in a literal sense. This is especially true then, as is often the case, the conductor is cable-in-conduit (CIC). In this configuration, the cable is sealed within a jacket, usually a stainless steel jacket. This jacket presses in on the cable, forcing the strands together. Between the temperature and the pressure, some sintering of the strands in inevitable.
The sintering and coupling problem has been recognized for some time, and several approaches have been taken to remedy it. In U.S. Pat. No. 4,330,347, Hirayama et al disclose a method by which to create an insulating layer on the copper surface of a conductor (see also "Cuprous Sulfide as a Film Insulation for Superconductors", G. R. Wagner, P. D. Vecchio, and J. H. Uphoff, Advances in Cryogenic Engineering (materials), Vol. 28, 1981). The conductor strand is baked in H.sub.2 S gas to create a thin (1.5-20 .mu.m) cuprous sulfide coating. An important feature of the invention is that no reduction in the conductivity of the copper occurs as a result of the sulfiding process (other than in the coating itself). The coating prevents sintering for 700.degree. C. temperatures held for as long as 30 hours. Inter-strand coupling is prevented by the coating up to a certain breakdown voltage. When this voltage is exceeded, the semiconducting coating freely passes current. Unlike the invention that forms the subject of this application, the cuprous sulfide insulator involves conversion of the copper conductor surface rather than the development of a distinctly separate metal coating.
In U.S. Pat. No. 4,990,491, Wagner et al. disclose a method by which to produce an insulating nickel oxide coating on a conductor strand surface. Nickel is plated onto the conductor and the conductor is then heated in an oxygen-rich atmosphere. The resulting coating has properties similar to cuprous sulfide but provides a much higher voltage stand-off and so is a more effective insulator. The patent states that no conductivity degradation occurs in the copper cladding of the strand as a result of nickel contamination for heat treatments of 700.degree. C. for more than 30 hours. Still, this is clearly a potential problem, as anyone knowledgeable in the art will understand. No metals other than nickel are disclosed as possible coating materials.
Another method by which to prevent inter-strand sintering for Nb.sub.3 Sn conductors is to plate chromium onto the strand surface. This is the method most commonly utilized in today's industry for large-scale projects like the proposed International Thermonuclear Experimental Reactor (ITER). Chromium plating does effectively prevent sintering and largely eliminates coupling (see, for example, "A.C. Losses in Multifilamentary Composite Superconducting Strands and Cables", T. M. Mower and Y. Iwasa, Advances in Cryogenic Engineering (Materials), Vol. 32, Edited by R. P. Reed and A. F. Clark, pp. 771-778, Plenum Press, New York, 1986), but the process has several drawbacks, including the following:
1. It is an expensive process. PA1 2. It is difficult to plate the chromium consistently over long lengths of strand. PA1 3. The bond of the plated chromium to the surface of the strand is often weak, resulting in flaking of the chromium during cabling operations. PA1 4. The hard chromium is poorly suited to the sharp bending involved in cabling. PA1 5. The copper and chromium can inter-diffuse during heat treatment, causing degradation of the copper conductivity and thereby undermining the electrical stability of the conductor.
A method for producing an insulating coating that is not subject to such problems and does not require special sulfiding or oxidizing steps would be of great use to industry. Such a method forms the subject of the present invention.
All of the above coating techniques, and the present invention as well, apply most particularly to Nb.sub.3 Sn conductors. The prior art contains numerous descriptions of fabrication methods for Nb.sub.3 Sn conductors. Examples include McDonald, U.S. Pat. No. 4,262,412, and Ozeryansky et al, U.S. Pat. No. 4,973,365. These patents disclose Nb.sub.3 Sn conductor fabrication by the bronze route and by the internal tin process, respectively.
Although the invention is most effective for cabled Nb.sub.3 Sn conductors, it may be applied to other conductors with equal ease. Cables utilizing NbTi conductor strands, for example, can also suffer coupling losses during a.c. operation, even though sintering is not a problem. The invention could be applied to increase the inter-strand resistance in such cable and thereby reduce the losses. That Nb.sub.3 Sn conductors are presented as the exemplar in the following discussion should therefore not be interpreted as a restriction upon the type of conductors to which the invention is applicable.